This Introduction originally appeared in:

During the 1960s, Chandler Robbins and his associates at the
Migratory Bird Population Station (now the PAtuxent Wildlife
Research Center) in Laurel, Maryland developed the concept of a
continental monitoring program for all breeding birds. The
roadside survey methodology was field tested during 1965, and the
North
American Breeding Bird Survey (BBS) was formally launched in 1966
when
approximately 600 surveys were conducted in the U.S. and Canada
east of the Mississippi River. The survey spread to the Great
Plains states and prairie provinces in 1967. By 1968,
approximately 2000 routes were established across southern Canada
and the contiguous 48 states, with more than 1000 routes surveyed
annually.

The BBS continued to grow as more birders became aware of the
program. During the 1980s, the BBS expanded into the Yukon and
Northwest Territories of Canada, and Alaska. Additional routes
have been added in a number of states. Today there are
approximately 3700 active BBS routes across the continental U.S.
and Canada, of which nearly 2900 are surveyed annually.

Breeding Bird Surveys are conducted during the peak of the
nesting season, primarily in June, although surveys in desert
regions and some southern states, (where the breeding season begins
earlier), are conducted in May. Each route is 24.5 miles long, with
a total of fifty stops located at 0.5 mile intervals along the
route. Here is an example of a BBS route located in Oregon.

A three-minute point count is conducted at each stop, during
which the observer records all birds heard or seen within 0.25
mile of the stop. Here is Chan Robbins demonstrating the roadside
survey technique of the BBS.

The BBS was designed to provide a continent-wide perspective
of population change. Routes are randomly located in order to
sample habitats that are representative of the entire region.
Other requirements such as consistent methodology and observer
expertise, visiting the same stops each year, and conducting
surveys under suitable weather conditions are necessary to produce
comparable data over time. A large sample size, (number of
routes), is needed to average local variations and reduce the
effects of sampling error, (variation in counts attributable to
both sampling technique and real variation in trends).

The density of BBS routes varies considerably across the
continent, reflecting regional densities of skilled birders. The
greatest densities are in the New England and Mid-Atlantic states,
while densities are lower elsewhere.
See map of route locations across North America.

Here is an example, showing route locations in Oregon.

Data are recorded at each stop, and then totaled over the entire
50 stop route. Once the data are recorded in the field, it is sent
to the BBS office at Patuxent where it is computerized. Here
is an example of a BBS field data sheet.

The BBS data are very challenging to analyze. The survey
produces an index of relative abundance rather than a complete
count of breeding bird populations. The data analyses assume that
fluctuations in these indices of abundance are representative of
the population as a whole.

Despite its complicated analyses, the BBS has proven to be a
very valuable source of information on bird population trends.
The following examples provide an indication of the types of
analyses that can be performed on these data.

BBS data can be used to produce continental-scale relative
abundance maps. When viewed at continental or regional scales,
these maps provide a reasonably good indication of the relative
abundances of species that are well sampled by the BBS. They
should be viewed with some caution, however. Where species approach
the edges of their ranges, they tend to be rare, locally
distributed, and likely to be poorly represented along BBS routes.
The procedures used to produce these maps also tend to distort the
edges of the ranges. Hence, BBS abundance maps provide only an
approximation of range edges. Another issue is that the precision
of the abundance estimate changes with sample size. Therefore,
estimates in regions with fewer routes are less precise
than estimates in regions with a large number of routes.
Here is an example of a BBS relative abundance map, for the Scarlet
Tanager.

Analyzing population change on survey routes is probably the
most effective use of BBS data, but these data do not provide an
explanation for the causes of population trends. To evaluate
population changes over time, BBS indices from individual routes
are combined to obtain regional and continental estimates of
trends. Although some species have consistent trends throughout
the history of the BBS, most do not. For example, populations of
permanent resident and short-distance migrant (birds wintering
primarily in the U.S. and Canada) species are adversely affected
by periodic episodes of unusually harsh winter weather. Examples
include the Carolina Wren whose populations dramatically declined
during the harsh winters of 1976-1977 and 1977-1978 but rapidly
recovered during the 1980s, as shown in the following graph.

Few species have consistent trends across their entire ranges,
so geographic patterns in trends are of considerable interest to
anyone concerned with the status of the continent's birds. Route-
specific trends can be smoothed to produce trend maps that allow
for the identification of regions of increase and decline. The
following map for Bobolink is typical of BBS trend
maps, and shows widespread declines in much of its range. Similar
declines have been experienced by most grassland birds.

Although trends at the species level will always be a basic
part of BBS data analyses, combining species into groups with
similar life-history traits, known as guilds, provides additional
insight into patterns of population trends. Consistent trends
within a guild may be indicative of the overall health of the
habitat factors defining the group, and establish a community
approach for analyses of BBS data.

Neotropical migrant birds have generated considerable interest
in recent years, although reports of their declines have been
exaggerated by some. The following map shows where the majority
of Neotropical migrants are increasing and where they are
decreasing, emphasizing the regional nature of the recent
declines within this guild.

With interest in the BBS at an all-time high among observers
conducting surveys and researchers using the data, its prospects
appear bright. Expansion in portions of central and western North
America will result in improved geographic coverage and more
accurate trend estimates for many species. We are also studying
the feasibility of making it a truly North American project by
expanding into northern Mexico. As part of a three-year study,
26 surveys were conducted by Mexican biologists in 1993
and 27 in 1994. The continued success of the BBS is a direct
result of the efforts of the state and provincial BBS
coordinators, and of the thousands of volunteers who conduct
surveys each year. These dedicated people deserve our thanks.